Powder metallurgy



Unit

This invention relates to metal powder compositions and products pressed and sintered from them.

In the production of iron articles by powder metal lurgical techniques it is common practice to add to the iron powder various other powders to develop special properties, such as strength, porosity, corrosion resistance or the like in the pressed and sintered articles. 11111218 been proposed to apply such methods to powderedfirofi substantially free from carbon mixed with from "0.2 5 to 1.0 percent of phosphorus. A disadvantage vexatious to fabricators of such mixtures is that the compacts undergo radial shrinkage during sinterin'gI. Suclr'ide crease in dimension is particularly, objectionable in that the obtention of parts of the desired dimensions is.

difficult and waste may be high due to the necessity to reject undersized parts.

It is among the objects of this invention to provide iron powder comnositions containing phosphorus and carbon in substantial amount which uponbeing formed into various articles bypressing and s'intering undergo reduced and unobjectionable shrinkage as compared with similar compositions substantially free from carbon, and which may actually exhibit a desirable amount of radial growth. g

It is another object of the invention to'pr'ovide iron powders according to the foregoing object which may be pressed and sintered without modification of existing practices in those operations.

It is a further object to provide a process of suppressing radial shrinkage of articles pressed and sintered from powder mixtures containing iron and phosphorus' This invention is predicated upon my discovery that the characteristic shrinkage of pressed and sintered articles of substantially carbon-free iron and 0.25 .to 1.0 percent of phosphorus can be controlled in a simple manner by introducing va definite but limited amount of carbon, preferably by adding carbon, most suitably as graphite, to the original powder mixture. The resulting articles, contrary to the usual result of evidencing objectionable shrinkage, are characterized by reducedand unobjectionable shrinkage, by substantial dimensional stability, or by an increase in the radialdimension according to adjustment of the original powder mixture.

Thus a major advantage of iron powder products made in accordance with this invention is that the shrinkage, or decrease in radial dimension, that occurswithmix: tures of carbon-free iron powders containingphosphorus is minimized with an actual increase in dimension being obtainable, if desired, or a substantially stable dimensior'iv is obtained. Attendant also upon the use of this invention is the obtention of these desirable results withoutthe necessity of using appreciable amounts of other expensive additive materials.

Iron powder mixtures in accordance with the pre ent invention comprise about 0.25 to 1.75 percent by weight of added carbon, preferably as graphite, about'0.2'5 to 1.0 percent by weight of phosphorus, and the balance substantially all substantially pure iron. Other elements .thesefFe-P powder compositions.

may,-,-of course,--be-ipresent so long as they do not deleteriously*afiect-theadvantageous results described; These materials; areusually present in impurity quantities and include, for-example, manganese, silicon, sulfur, nickel, copper vanadium, aluminum, and "the like; Where silicon ispreseng-its amount should be limited toia maximum of 0.4 percent, preferably,Q.3{percent,;.by weightof the total mixture, These mixturesare then-pressed and sinteredgaecording toconventionalpractice.

;;Phospho rus is generally present in t-he powder in amounts of:ab out .25 to-;: 1 O,,percent :byw tight. The phosphorus to most advantage is introduced as ferrophosphorus, for example as fend-phosphorus containing about to percent by weight of phosphorus, and preferably? ferrwphosphorus analyzing at about 24 to 26 percentphosphorus. 1 A V Products ,produced in accordance with the present invention are not characterized by theiexcessive brittleness that has been ascribed to the inclusion of carbon in .Also, ductility and sintered strength and similar properties are consistently satisfactory in the articles ofthisinvention.- Certain combinations of properties are'ch aracteristic of each specific composition with the described ranges and the development of otherspecialproperties can be obtained through judicious use of additional components. -By way of illustration, copper 'pewderin amounts of about 3 to 10 .percent .by weight ofthe tot l compos t on cub n: l eluded. Copper is known to limit shrinkage and even producesanincrease'in'themodulus of rupture, ultimate tensile strength'and hardness-ascornpared with similar compositions containing no copper. Where the additional cost ofthe copper'powderis' not of pressing 'Significancejits use may be indicated-to obtain-the'further improvement'in properties just-described.

In practicingthe invention, a typical powder mixture that 1 have used with satisfaction is an iron powder mixture containing about 2.50 percent by weight of a 250mesh ferro-phosphorus having a phosphorus content of 24 to 26 percent by weight. The mixture has the following chemical and physical characteristics:

g Talile I Chemical data Hydrogen loss,-percent j 0.72 "Total carbon, percent 0.25 Acid insolubles, percent '0.'25

"Ferrophosphorus; 24-26% and Z-SO- 'mesh,

percent 9 2.50

Physical data -Apparent-density(Hall) a 2.62 Flow (Hall) 28.60 Screen, percent- On 0.40 On 14,80 On 4.30 on 28150 01 200 f r 17.00 011325 21.70

TJ:';;=' Z.

1 All percentages mare iby weight: based on the total" compos on.

To samples of this mixture quantifies of graphite were intimately admixed. The mixtures were then placed in steel dies, pressed at 30 t.s .i. and sintered in hydrogen for 30 minutes at 2020 F. One percent by weight of zince stearate was added to all of the powder mixtures Upon comparing any of compositions 11 through 16 with composition 1, the reduced shrinkage attending the addition of carbon to iron-phosphorus powder mixtures will be apparent. The measure of this reduction varies with the type and amount of graphite added. Thus, 0.75 percent of graphite A resulted in growth while that amount of graphite B iri a similar compact resulted in shrinkage, as may be seen by comparing compositions 13 and 14, or by comparing compositions and 16. By using a mixture of different graphites, the growth or shrinkage of the resultant compact may be intermediate that obtained upon using an equal amount of either graphite alone.

According to the provisions of the patent statutes, I have explained the principle of my invention and have described what I now consider to represent its .best em- Table II Modulus Change Ultimate Elongation Composition Of In Radial Tensile In One Hardness,

Rupture, Dimension, Strength, Inch, RB

p.s.i.' Percent 1 11.8.1. Percent 1. Mixture A 1 80, 000 95 36, 000 2. 00 48. 0 2. Mixture A +.50% graph1te. 68, 000 32, 000 1. 75 45. 0 3. Mixture A +.75% graphite 74, 000 16 000 1. 60 46. 0 4. Mlrture A +10% graphite 75, 000 80 37. 500 1. 50 48. 0 5. Mixture A +1.51% graphite" 66, 500 36 32, 600 1. 50 52. 0 6. Mixture A +30% copper--- 86, 000 05 42. 300 1. 50. 0 7. Mixture A copper +.50% graphite.-- 84, 000 25 43. 000 1. 00 51. 0 8. Mixture A +3.0% copper +.75% graphite--- 92; 000 44, 400 1. 25 55. 0 9. Mixture A +30% copper +1.0% graphite-.- 94, 000 45, 300 1. 25 58. 0 10. Iron Powder alone 40, 125 16 16, 500 4. 00 18. 7 RE 1 Mixture A in all cases is iron powder +2.50% FeP. 14 Minus indicates shrinkage; plus indicates growth.

It will be observed upon comparing compositions 1 and 2 that the inclusion of carbon materially reduces the radial shrinkage of pressed and sintered iron-phosphorus powder mixtures. Indeed, in the upper ranges of carbon, significant growth was obtained (compare compositions 3, 4 and 5 with 2). In compositions 6 through 9, copper was included. By comparing compositions 7 with 2, 8 with 3, and 9 with 4, the efiect. of the conjoint use of copper and carbon can be observed. '1 It can also be observed that compositions containin g copper have the better properties as regards modulus-of rupture, ultimate strength and hardness. Surprisingly, the use of carbon usually increases the value of these properties when copper is present rather than producing the expected decrease. This can be observed by comparing compositions 7, 8 and 9 with 6.

bodiment. However, I desire to have it understood that, within the scope of the appended claim, the invention may be practiced otherwise than as specificallydescribed.

I claim: i

That method of suppressing the radial shrinkage of articles manufactured by pressing and sintering, in an atmosphere consisting essentially of hydrogen, a mixture of powders consisting essentially of iron powder and about 0.25 to 1.0 percent by weight of phosphorus which comprises adding to said mixture sufiicient elemental carbon to result in a carbon content of the pressed andsintered article of about 0.25 to 1.75 percent based onthe total composition.

References Cited in the file of this patent UNITED STATES PATENTS A further measure of control of shrinkage attends the 2,229,330 Langhammel' 6t 1941 choice of carbon added to the iron and phosphorus FOREIGN PATENTS powder mixtures, for certam graphites have a more 456,948 Canada May 24,1949

pronounced effect than others. This was demonstrated by preparing standard bars from powder mixes while varying the quantity and type of graphite. Two commercially available graphites were used. The data obtained are:

OTHER REFERENCES Table III Modulus Change Ultimate Elongation Composition 0f In Radial Tensile In One Hardness,

Rupture, Dimension, Strength, Inch, RB

p.s.i. Percent p.s.i. Percent 11. Mixture A 1 +.50% graphite I- 67, 000 .25 31.000 1.00 44.9 12. Mixture A +.50% graphite II. 67, 500 24 28, 600 1. 00 41.8 13. Mixture A +.75% graphite L- 68, 600 18 32. 700 1. 00 42.1 14. Mixture A +15% graphite II- 68. 300 18 80. 600 50 40. 2 15, Mixture A +1.0% graphite L--- 73, 600 35 37. 500 1. 00 42. 4 16. Mixture A +1.13% graphite II 65, 000 01 29,050 50 44. 9

1 Mixture A in all cases is iron powder +2.50% FeP. 8 Minus indicates shrinkage; plus indicates growth. 

